The exemplary embodiment relates to an illumination apparatus which includes a light source including a solid state light emitting device (SSL) such as an LED, laser diode, or organic light emitting device (OLED). It finds particular application in connection with a modulated current source which reduces the observed variation in color temperature which can occur when the light source includes a phosphor material.
Light emitting diodes (“LEDs”) are well-known solid state lighting devices that can generate light having a peak wavelength in a specific region of the light spectrum. LEDs are typically used as illuminators, indicators and displays. LEDs based on a gallium nitride (GaN) die have been developed that can efficiently emit light having a peak wavelength in the blue and/or UV region of the spectrum. To produce white light (light with a relatively uniform intensity across the visible range) or other selected light color, the LED is often combined with a phosphor material which includes a phosphor or group of phosphors. Each of the phosphors converts some of the light emitted by the LED to light of a different, generally longer, wavelength using a process known as phosphorescence.
Numerous phosphors and phosphor combinations have been developed for this purpose, as disclosed, for example, in U.S. Pat. Nos. 5,998,925; 6,522,065; and 6,538,371; and U.S. Pub. Nos. 2008/0135860 and 2008/0124999. For example, a phosphor comprising (Y0.4Gd0.6)3Al5O12:Ce is used to convert a portion of a blue emitted light from a GaN die to yellow light. The total emission of the LED device, consisting of the blue GaN device generated light and yellow phosphor emitted light is a generally white light.
An LED has a relatively fast response to the input current, turning on quickly when the current is applied and turning off quickly when the current is switched off. However, the phosphor exhibits a different response. While the phosphor turns on (starts phosphorescing) relatively rapidly in response to the emitted LED light, it has a relatively long decay time. Moreover, the color of LED devices tends to shift over the design lifetime. This means that the color emitted by the illumination apparatus is not consistent over time.
Various attempts have been made to provide consistent color in LED devices. In one approach, multiple phosphors have been used, having different decay times. This requires careful control of the phosphor proportions. In other approaches, multiple LEDs with different emissions have been used, which together generate a desired color, without the need for phosphors. However, the control circuitry for such an arrangement tends to be complex.
Thus, a need exists for a method for achieving a selected color from a light source, such as one that includes a solid state light emitting device in combination with a phosphor material.